The present invention relates to a coextruded and biaxially stretched, opaque polyolefin film having improved mechanical properties, which comprises a filler-containing opaque polyolefin support film and two transparent or clear polyolefin covering layers which cover the two surfaces of the support film. The invention also relates to a process for the production of such films and to the use thereof.
U.S. Pat. Nos. 4,303,708 and 4,377,616 both disclose a coextruded and biaxially stretched, opaque polyolefin film which comprises an opaque support film and two transparent or clear outer layers (covering layers). The support film which has a considerably greater thickness compared with the covering layers, is a monolayer film and comprises a polyolefin and fillers. The opacity of the support film results from the fact that, owing to the filler content and biaxial stretching, a great number of voids (vacuoles) are present. In the stretching operation, the polymer matrix is torn open at the grain boundaries of the inorganic or organic fillers which are incompatible with the polyolefin, such that the above-indicated vacuoles, i.e. free (empty or unfilled) spaces, are formed. It is a matter of course that these vacuoles or micro-cavities are distributed throughout the volume of the support film. The refraction (scattering) of light at the voids imparts an extremely attractive opaque appearance to the film. Since these multi-layer polyolefin films are especially used in the field of packaging, it would be advantageous for them to also possess good mechanical properties. However, in this respect the films are still unsatisfactory. This applies, in particular, to opaque polyolefin films of low density, i.e. having a density of less than 0.9 g/cm.sup.3.
From published European Patent Application No. 0,079,520 it is, moreover, known to use so-called hard resins which comprise natural or synthetic resins of various types having softening points of at least 70.degree. C., as additives in polyolefins. The resin-modified polyolefin films described in this publication concern light-transmitting, rather than opaque films.